(1) Field of the Invention
This invention relates to a method and apparatus for dry process fabrication of devices from a substrate. In particular, a substrate is exposed to the effluent of a gas plasma having at least one reactive specie, but being substantially free of electrically charged particles, the substrate being simultaneously irradiated with ultraviolet radiation to enhance the reaction rate in a controlled manner. The substrate may also be heated by infrared radiation.
(2) Description of the Prior Art
The use of a gas plasma in the fabrication of solid state devices is known in the prior art. Plasmas have been used both in the etching of semiconductor substrates and in the stripping or removal of photoresist layers from substrates.
Plasma processing, also known as dry processing or RIE (reactive ion etching), has several distinct advantages over the more conventional processing, known as chemical or wet processing. Wet processes typically use toxic compositions to remove photoresists or to etch a layer or layers of material from a substrate. Some chemical compositions may be hazardous to the individual operator and to the environment if not carefully used. Waste disposal from wet etching processes can also present a problem.
Wet etching is isotropic in that etching proceeds at a substantially equal rate in all directions, thereby leading to an etch that extends not only downwardly in the desired direction but laterally as well. Isotropy produces an undesirable undercutting effect, reducing the distances between adjacent etches to often unacceptably small distances. If line widths, or the lateral distances between adjacent etches, must be held within very small tolerances such as those required for the small geometries of many LSI or VLSI devices, wet etching may not be usable. In many of these devices, the line tolerances are often comparable to the thicknesses of the films being etched, and anisotropy is therefore essential.
The isotropic etching that results from wet processes has become more unacceptable to the industry as the density of circuit elements placed on a single semiconductor substrate has increased. As element density increases, the line widths decrease, and isotropic etching becomes more unacceptable. Accordingly, the need for anisotropic or straight-walled etching has increased.
As compared to wet etching, dry etching provides the capability of anisotropic etching for holding line widths within specified tolerances. In the typical dry etching apparatus, the semiconductor wafers or substrates being processed are placed in a plasma etching chamber directly within the plasma or glow discharge region, where electrically charged particles and relatively strong electric fields are present. The presence of charged particles within the region of a strong electric field achieves anisotropy in the etching process, because the electric field imparts directionality to the charged etching species. Precise device fabrication is thereby possible.
However, the semiconductor layers on LSI or VLSI devices, such as for example a dielectric layer of silicon dioxide, can be relatively thin, perhaps on the order of 1 micron, and ion bombardment from the charged particles being accelerated into the layer by the electric field can result in electrical anomalies or unacceptable damage to the layer, known generally as radiation damage. As oxide layers decrease in thickness, the number of defective chips and reduction of chip yields per wafer caused by radiation damage increases to unacceptable proportions.
Radiation damage is likewise a problem in dry photoresist strippers, and is not a problem limited only to plasma etching. Also static charge builds up on the surface of exposed dielectric layers. For example, EPROM devices normally incorporate a dielectric oxide layer only 100 angstroms thick. Static charging of the dielectric layer during dry stripping of photoresist can result in dielectric breakdown and resultant inoperability of the device.
Dry etching devices wherein the substrate being etched is removed from the plasma itself, i.e. a "downstream" etching apparatus, is known in the art. However, while the problem of radiation damage is reduced, the typical downstream device suffers from the same inadequacy of the wet etching devices in that etching is unacceptably isotropic, because in the downstream device there is typically no electric field to impart any directionality to the etching species.
U.S. Pat. No. 4,233,109 to Nishizawa discloses a plasma etching method using a plasma generator to ionize a reaction gas into a plasma state. The generator is connected to a processing chamber by a nozzle for introducing the plasma into the chamber, which contains a workpiece to be processed.
It is also known to use photons to affect the action of a plasma reactor. U.S. Pat. No. 4,183,780 to McKenna et al. describes a method and apparatus for photon enhanced reactive ion etching, wherein the plasma reactor includes means for emitting selected wavelengths of vacuum ultraviolet and directing this radiation to the plasma, preferably adjacent the substrate to control the plasma process, especially at the substrate. U.S. Pat. No. 4,404,072 to Kohl and U.S. Pat. No. 4,478,677 to Chen et al. disclose the use of light in etching methods.
However, none of these references either singly or in combination show a downstream plasma reactor or method that combines reduction or avoidance of radiation damage with the capability of directionality for the active species, which is desirable in the fabrication of LSI or VLSI devices.